What this PR does / why we need it:
在pgxc_planner中会优先生成stream计划，当stream计划生成失败后，再尝试pgxc计划。在生成stream计划的时候，创建了一个临时ResourceOwner，计划生成完成后，切换了ResourceOwner，但没有将这个临时的ResourceOwner释放掉，导致了内存泄漏。在切换ResourceOwner时，增加临时ResourceOwner的释放操作，避免内存泄漏。

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

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